Communication systems that utilize coded communication signals are known in the art. One such system is a direct sequence code division multiple access (DS-CDMA) cellular communication system, such as set forth in the Telecommunications Industry Association Interim Standard 95A (TIA/EIA IS-95A) herein after referred to as IS-95A. In accordance with IS-95A, the coded communication signals used in the DS-CDMA system comprise spread spectrum signals that are transmitted in a common 1.25 MHz bandwidth channel between mobile stations and base transceiver stations (BTS) located at base sites of the wireless communication system. Each 1.25 MHz bandwidth portion of the radio-frequency (RF) spectrum, or 1.25 MHz bandwidth channel, carries spread spectrum signals centered around a particular carrier frequency and is commonly referred to as a narrowband DS-CDMA channel.
A communication signal transmitted in a narrowband DS-CDMA channel between a mobile station and a base site, is reflected off of nearby scatterers, such as buildings resulting in multipath propagation of the transmitted signal. These reflections produce replicas of the originally transmitted signal which arrive at a receiver, such as a base station receiver or a mobile station receiver, with various power levels at various times, depending on the effective propagation distances traveled by the replicas as well as signal interference (a.k.a. noise). Upon receipt by a receiver, the originally transmitted signal and its reflected replicas are filtered, correlated, despread, recombined and then decoded, within their applicable narrowband DS-CDMA channel.
Currently, there are numerous international standards proposals which suggest a wideband CDMA transmission signal configuration. Evolution from the current narrowband CDMA signal transmission configuration, which is designed to carry voice and low speed data, has been driven by a need to deploy higher rate voice and packet data services in a wireless communication environment. Two wideband CDMA signal transmission configurations have been described, a multi-carrier CDMA signal transmission configuration and a direct-spread CDMA signal transmission configuration. In the multi-carrier signal transmission carrier configuration, three 1.25 MHz narrow-band CDMA signals are represented in a 5 MHz bandwidth. Consequently, a communication signal centered at a particular carrier frequency may be transmitted at a maximum of 1.2288 mega chips per second (Mcps). Each 1.25 MHz carrier is approximately orthogonal to the others, thus providing the necessary orthogonal diversity paths when the carriers are transmitted from different antennas. Similarly, six 1.25 MHz carriers may be used in a 10 MHz bandwidth and twelve 1.25 MHz carriers may be used in a 20 MHz bandwidth. In the direct-spread CDMA signal transmission configuration, a communication signal is spread across an RF bandwidth equivalent to multiple narrowbands, for example, three 1.25 MHz CDMA carriers subsequently transmitted at 3.6864 Mcps. At the receiving end, the direct-spread signal is demodulated and converted to a digital signal stream, much like narrowband CDMA execution.
Unfortunately, when a wideband DS-CDMA communication system is overlaid across RF spectrum defined for multiple narrowband DS-CDMA communication system signals, conventional receiver front end processing, which uses only one AGC circuit to gain control the received signal before further processing, is inadequate. Specifically, when the multiple narrowband signals comprising the wideband, have unbalanced noise density with respect to each other, differing interference will be present in the bands. As a result, the signal-to-interference ratios (SIR) of the bands may be different. As a result of the varied SIRs, the prior art use of a single AGC for gain control in the receiver front end may yield non-optimal performance of the receiver.
Therefore, a need exists for a method and apparatus to provide receiver front end processing for a wideband communication signal which is easy to implement and overcomes signal gain control problems inherent in the prior art design.